Photoconductive titanium dioxide material

ABSTRACT

A NEW TYPE OF PHOTOCONDUCTIVE ANATASE TITANIUM DIOXIDE COMPOSITION HAS BEEN PRODUCED WHICH COMPRISES EUHEDRAL SHAPED TITANIUM DIOXIDE PARTICLES HAVING AN AVERAGE INDIVIDUAL CRYSTAL SIZE FROM 0.2 TO 5.0 MICRONS, THE SURFACE OF SAID CRYSTALS CONTAINING A DENSE COATING OF ZINC OXIDE IN AMOUNT FROM 1% TO 4% AND CONTAINING FROM 0% TO 1% OF A DENSE COATING OF LEAD OXIDE, THE PERCENTAGES BASED ON THE WEIGHT OF THE TITANIUM DIOXIDE.

Patented Oct. 17, 1972 3,698,894 PHOTOCONDUCTIVE TITANIUM DIOXIDE MATERIAL Warren M. Foss, Metuchen, N.J., assignor to NL Industries, Inc., New York, N.Y.

No Drawing. Filed Aug. 20, 1970, Ser. No. 65,724 Int. Cl. C03g 5/00, 7/00 US. Cl. 96-15 4 Claims ABSTRACT OF THE DISCLOSURE A new type of photoconductive anatase titanium dioxide composition has been produced which comprises euhedral shaped titanium dioxide particles having an average individual crystal size from 0.2 to 5.0 microns, the surface of said crystals containing a dense coating of zinc oxide in amount from 1% to 4% and containing from to 1% of a dense coating of lead oxide, the percentages based on the weight of the titanium dioxide.

BACKGROUND OF THE INVENTION Many types of photoconductive pigments have been produced by the prior art. The instant invention is concerned with the preparation of photoconductive titanium dioxide pigments. In application Ser. No. 1,529 filed Ian. 8, 1970, now U.S. Pat. No. 3,632,527, a process is described for producing a new type of photoconductive TiO material which may or may not contain loosely adhering coatings of A1 0 SiO ZnO, PbO and TiO The TiO material itself is described in US. Pat. No. 3,632,527 and consists of euhedral shaped crystals having an average crystal size of 0.2 to 5.0 microns. Although this material possesses a high degree of photoconductivity when incorporated in a paper system, whether the TiO-,, particles are coated or not, the photoconductivity is increased if the particular type of dense, firmly adherent zinc oxide coating of the instant invention is formed on the surface of the euhedral Ti0 particles. In some instances the photoconductive properties may be still further enhanced if a dense coating of lead oxide is also applied.

SUMMARY OF THE INVENTION The photoconductive product of the instant invention consists of euhedral shaped crystals of anatase titanium dioxide having an average particle size of 0.2 to 5.0 microns, the surface of said particles having a firmly adherent, dense coating of zinc oxide in amount from 1% to 4% and containing in addition, if desired, a dense coating of lead oxides in amount from 0% to 1%.

Basically the euhedral shaped crystals of TiO are prepared using the process described in US. Pat. No. 3,632,- 527 and the crystals are coated in the following manner.

The Ti0 particles after ball-milling are slurried in water and the slurry is heated to 60 C. To the heated slurry is added an alkali metal hydroxide until the pH is raised to 10-11. A solution of a zinc salt, such as e.g. zinc sulfate, is then added over a period of l5-30 minutes and the pH is held above 10. If desired the lead oxide coating is also applied by adding a soluble lead salt, such as e.g. lead nitrate, at the same pH.

After holding the mixture for at least 30 minutes at a pH of at least 10, the pH is lowered slowly to pH of 7.5 in an orderly fashion over a period of one hour. The product is then filtered and washed well with water. After drying at 110-120 C., the product is then thoroughly milled.

DESCRIPTION OF THE PREFERRED EMBODIMENT The TiO base composition used in the instant invention is prepared by the process described in US. Pat. No. 3,632,527 which comprises hydrolyzing a titanium sulfate-iron sulfate solution to form a titania hydrate, filtering, bleaching and washing said hydrate to remove the soluble iron salts therefrom. The bleached hydrate is substantially iron-free but contains from 5% to 15% H 50 associated with said hydrate. The hydrate is slurried and treated with a suflicient amount of an ammoniacal agent selected from the group consisting of ammonia, ammonium hydroxide and ammonium carbonate to neutralize the titania hydrate slurry to a pH of 5.0 to 11.0. The neutralized titania hydrate contains an amount of sulfate no greater than 2% in said hydrate, calculated on a Ti0 basis. The washed titania hydrate is treated with an alkali metal salt selected from the group consisting of sodium chloride, sodium sulfate, sodium pyrosulfate, sodium hydroxide, sodium carbonate, potassium chloride, potassium sulfate, potassium pyrosulfate, potassium hydroxide and potassium carbonate. The amount of sodium compound added, calculated as Na O, is from 0.4% to 2.0%, while the amount of potassium compound added, calculated as K 0, is from 0.4% to 3.0%. The hydrate should contain no more than 0.1% P 0 on a TiO basis. The treated hydrate is then calcined at a temperature from 900 C. to 1025 C. for hydrates containing K 0 and 875 C. to 925 C. for hydrates containing N21 O to develop the highly crystalline anatase titanium dioxide euhedral material. The calcined material is then ground and the ground material has an average individual crystal size of from 0.2 to 5.0 microns. The size of the individual crystals are about 0.2 micron when the K 0 treated hydrate is calcined at 900 C. This size increases however to about 0.7 micron when the calcination temperature is raised to 1000 C.

Using Na O as the treating agent, the titanium dioxide crystals have the anatase structure when calcined at about 875-925 C. with the crystal Size of about 0.25-0.5 micron.

The use of potassium salts is preferred over sodium salts since it is easier to maintain the crystal structure of anatase using potassium salts. Potassium sulfate is particularly desirable to employ since its use reduces the amount of potassium titanate formed. It is desirable to maintain the amount of potassium titanate formation to less than 2%. The rutile content preferably should also be held below 10%.

In the previously described patent application Ser. No. 1,529 the euhedral TiO particles may or may not be coated with a metal oxide coating of A1 0 SiO ZnO, PbO or TiO This coating is formed on the surface of the TiO particles as a loosely adhering coating. It is formed by adding an aqueous solution of an acid salt of the metal compound to a slurry of the TiO and the metal oxide is formed by reacting the acid salt of the metal compound with an alkali (e.g. NaOH) which precipitates the metal oxide compound onto the surface of the TiO; particles. The pH of the final slurry is about 7.0.

In contrast to the previously described process in US. Pat. No. 3,632,527 the euhedral TiO particles in the instant invention are coated in a different manner with ZnO and PbO and therefore a firmly adherent and dense coating of ZnO and PbO is formed on the surface of the TiO particles. The instant coating process may be described as follows:

(1) a ball-milled slurry of euhedral Ti0 particles are heated to 60 to C. and the pH is adjusted to at least 10 to 11 by adding an alkali metal compound, e.g. NaOI-I;

(2) a zinc salt, e.g. ZnSO is then added as an aqueous solution to the Ti slurry over a period of at least 15 minutes while the pH is held above 10.0;

(3) after the zinc salt has been added a lead solution may then be added if desired;

(4) the mixture was then held for at least 30 minutes at a pH of at least 10;

(5) after all of the metal salts had been added the pH was lowered at the rate of 0.5 pH units per minute intervals to pH of 7.5 by adding dilute H 80 to the mixture.

After coating the product is filtered, washed, dried and ground thoroughly.

The final products are tested for photoconductivity as follows:

TESTS FOR PHOTOCONDUCTIVITY The titanium dioxide material is dispersed in a solution of methyl methacrylate copolymer made by E. I. du Pont Company and sold under the name of Elvacite 6014. To the mixture is added 0.02% fluorescein from a methanol solution.

25.0 grams of the titanium dioxide material are dispersed in 62.5 mls. of a solution of the copolymer (100 grams copolymer solution diluted to 1000 ml. with toluene) by mixing in a high speed blender for 5 minutes. The mixture is then applied to an aluminum sheet 0.0015 inch thick with a Bird film applicator (gap width 0.003 inch, wet film thickness of about 0.0015 inch) to give a coating weight of about 13 pounds per 3000 sq. ft. per ream. The film is then over-dried at 50 C. for one minute.

A small piece of the coated foil is then dark-adapted overnight and tested in the dynamic-capacitor electrometer where its electro-photographic properties are measured as follows:

1-charge acceptance 2dark decay 3light decay The dynamic-capacitor electrometer used is similar to the apparatus described in R.C.A. Review, 22, 780-90 (1961), except that the one square inch samples are mounted on the rim of the nine inch diameter wheel (30 in. circumference) so that the true charging time and light exposure times are each l/30' the nominal time in the cycle. The negative corona is operated at 7000 volts. The probe voltages are followed with a strip chart recorder. The light source is a 500 watt tungsten-iodine filament lamp mounted 8 inches from the edge of the rim with a 10% neutral density filter to reduce the light intensity to about 25 foot-candles at the specimen. The cycle arbitrarily selected for the test involves 10.0 seconds (nominal) of charging, 5.0 seconds of dark decay and the light exposure is continued to allow the surface voltage to decay below 40 volts.

For a titanium dioxide material to be acceptable for the copy paper industry the charge acceptance should be at least 300 volts within a ten-second period, the dark decay less than 60% of the charge acceptance, the light decay at least 70 volts per second and it should have a residual of no more than 40 volts. Typically an acceptable film shows a charge acceptance of 320 volts, a dark decay of 20 volts per second, a light decay of 70 volts per second.

The product produced is then further tested for useful photoconductivity by actually preparing a print on a test film in a commercial copier. For this test, the pigmented copolymer film prepared as described above and coated either on a 3 mil. aluminum foil or a commercial- 1y available conductive paper, is dark-adapted overnight and used to make a single page copy on a commercially available copier. For this purpose a Model 33 Electrostatic Copier manufactured by the SCM Corporation is used. A useful photoconductive film will yield a high contrast, clear and sharp copy with a light background and little or on streaking, stemming from inhomogeneous corona charging of the test film, in the direction of motion of the film through the machine.

The following examples are presented in order to describe the instant invention in more detail:

EXAMPLE 1 Euhedral Ti0 particles were prepared as follows:

3600 mls. of a titanium sulfate-iron sulfate solution containing 250 g.p.l. TiO 169 g.p.l. FeSO and 500 g.p.l. H 50 were heated to 96 C. and were added within a period of 16 minutes to 900 mls. water heated to 96 C. The entire mixture was heated to boiling and boiled for 3 hours to complete the hydrolysis. 950 mls. of hot water were added to the mixture to cut the concentration to 165 g.p.l. Ti0

The hydrate was filtered, washed free from soluble iron salts, bleached with 10% H and 0.1% aluminum metal for 1 hour at 80 C. at 20% solids, then filtered and washed with water until iron free. The washed filter ca'ke contained 10% H 80, on a TiO basis. 2570 grams of the filter cake containing 35% solids were admixed with 1500 mls. water to form a slurry containing 22% solids. With agitation, 31.5 grams of ammonia, added as a solution in water, were added to the hydrate slurry to neutralize substantially all of the sulfuric acid present in the hydrate. The pH of the neutralized hydrate was 7.5. The neutralized hydrate was heated at 60 C. for 1 hour and the pH readjusted with ammonia to a pH of 7.5.

The neutralized solids were then allowed to settle and the solids content were washed with 70 liters of water containing 5 g.p.l. NH Cl. The washed titania hydrate contained 0.18% sulfate calculated as S0 and 0.07% P 0 on a TiO basis.

The washed titania hydrate was then treated with KOH, calculated at 1.25% K 0 on a TiO basis, and the treated hydrate was then rotary calcined at 990 C. for 3 hours to produce a highly crystalline anatase titanium dioxide material. The calcined material contained 1.25% K 0, 0.07% P 0 and the individual crystals of the Ti0 were euhedral with a major portion of the crystal edges being straight-sided. The average individual crystal size was 0.7 micron.

These euhedral TiO particles were then coated as follows:

( 1) 150 pounds of the euhedral TiO crystals described above were admixed with water to form a slurry containing 33% solids; 0.2% of a 'dipsersing agent, such as monoisopropanolamine, were added to the slurry and the slurry was heated to 60 C.;

(2) to the heated slurry were added a 25% solution of sodium hydroxide to raise the pH to 10.5 after which another 2.5% additional NaOH (Ti0 basis) were added;

(3) after holding for 15 minutes, 27.0 liters of ZnSO solution containing g.p.l. ZnSO were added over a period of 20 minutes. This amount of ZnSO is equal to 2% ZnO on a TiO weight basis. The pH of the solution was maintained at pH of 10.5 during the addition period;

(4) after all of the ZnSO has been added the slurry was maintained at pH of 10.5 and 60 C. for 30 minutes;

(5) a 10% H 80 solution was added to lower the pH to 10.0. The slurry was maintained at this pH for 10 minutes;

(6) the pH of the slurry was then lowered 0.5 pH at 10 minute intervals until the pH of the slurry reached 7.5;

(7) after holding at pH of 7.5 for 1 hour the slurry was filtered and the cake washed with H O until sulfate free;

(8) the washed filter cake was then dried and micropulverized.

adding an acid until the pH is lowered to at least 7.5, then filtering, washing and drying said coated product, thus The ground product has the following photoconduc- We pr perms forming said dense adherent zinc oxide coatlng on said Charge ceptance 364 euhedral titanium dioxide crystals. Dark decay Q3 5 2. Composition of matter consisting essentially of Light decay 10 euhedral titanium dioxide crystals, a majority of the edges EXAMPLES of said crystalis lasing straitglllt-sided% :aid c(r)y2stals shaving an average in 1V1 ua crys a size 0 rom to m1- In these examples h euhedral 2 prodyucedm crons, said crystals having the crystal structure of anatase, amplh 1 Was cqated Wlth and 05% 115mg h said crystals being coated with a dense adherent coating prochfhlre deschlbed 111 3,632,527 wheteln of zinc oxide and in addition coated with a dense adherent the T10 slurry 1s treated with zinc sulfate and neutralized coating f lead oxide, the amount f Zinc oxide coating with, N to PH of The oxide l being from 1.0% to 4.0%, and the amount of lead oxide precipitated onto the surface of the T10 The coati g 13 coating being from 0.0% to 1.0%, the percentages based not firmly adherent 15 on the weight of said titanium dioxide, said composition The P Pmpemes of theise examples are prepared by forming an aqueous slurry of euhedral TiO recorded in the following table along with those of Exparticles, heating the slurry to at least 10 by adding an ample alkali metal compound, adding a soluble zinc salt to said EXAMPLE 4 aqueous slurry over a period of at least 15 minutes while [In this example the euhedral TiO particles were coated maintaining the p to at least 10, adding a soluble lead in the same manner as that described in Example 1 ex- Salt to Said aqueous ShlfTY Over a Period of at least 15 cept after Step 3, 5.05 liters of lead nitrate solution conminutes While maintaining the P to at least holding taining 100 g.p.l. PbO were also add d ft th Z SQ, the mixture for at least 30 minutes at a pH of at least addition. The amount of lead nitrate added was equal 1 then lowe ing the pH at a rate of no more than 0.5

to 0.5% PbO on a TiO basis. The ZnO amount remained PH units P 10 minute intervals y adding an acid Until at 2.0%. The coated Ti0 was further processed in the the pH is lowered to at least 7.5, then filtering, washing same manner as Example 1 and the ground product as and drying said coated product, thus forming said dense tested for photoconductivity. The results are also recorded adherent coatings of zinc oxide and lead oxide on said in the following table. euhedral titanium dioxide crystals.

5M 3. Method for producing a new photoconductive com- EX PLES 5H8 position of matter consisting essentially of euhedral tita- In these runs the procedure of Example 1 was repeated nium dioxide anatase crystals, said crystals containing an except that the amount of zinc oxide coating was varied adherent dense coating of zinc oxide, said process which from 1.0% to 4.0%. The photoconductivity results are comprises forming an aqueous slurry of euhedral TiO recorded in the table. particles, heating the slurry from 60 C. to 90 C., adjust- TABLE Examples 2110, percent 2.0 1.0 0.5 2.0 1.0 2.0 3.0 4.0. PbO, percent. 0.5 Photoconduetivity:

Charge acceptance- 364"; 420..-- 384. 344 428 388 348.. Dark decay.--" 23 20 21.. 21 29 29-..

Light decay".-- 100 76 125 l0 111 107 98.

Type of coating- Dense, firmly Fluffy, loosely Fluffy, loosely Dense, firmly Dense, firmly Dense, firmly Dense, firmly Dense, firmly adherent. adherent. adherent. adherent. adherent. adherent. adherent. adherent.

It has clearly been shown by the table that the dense ing the pH of the slurry to at least 10 by adding an alkali adherent coating of ZnO formed by the process of the inmetal compound, adding a soluble zinc salt to said aquestant invention onto the surface of the euhedral TiO ous slurry over a period of at least 15 minutes while mainparticles produces superior photoconductivity to that protaining the pH to at least 10, the amount of said zinc salt duced by the process described in US. Pat. No. 3,632,527 being from 1.0% to 4.0% calculated as zinc oxide and which forms a loosely adhering Zn-O coating. based on the weight of said titanium dioxide, holding the While this invention has been described and illustrated mixture for at least 30 minutes at a pH of at least 10, by the examples shown, it is not intended to be strictly then lowering the pH at a rate of no more than 0.5 pH limited thereto, and other variations and modifications unit per 10 minute intervals by adding an acid until the may be employed within the scope of the following claims. P is lowered to at least then filtering, Washlhg and 1 claim; drying said coated product.

1. Composition of matter consisting essentially of Method for P f h a new Photoconductlve Q euhedral titanium dioxide crystals, a majority of the edges h h matter conslshhg esshhtlally 0f euhedf of said crystals being straight-sided, said crystals having mum dloxlde ahatas? ry t ls, Sa1d crystalscontarn rng an an average individual crystal size of from 0.2 to 5.0 miadherent dense coatmg of Zmc oxlde and m addmon an crons, said crystals having the crystal structure of anatase, dense Foatmg of lead oxide sald Process which comprises forming an aqueous slurry of euhedral T10 Said. crysttls being coated i a i adherent. coating particles, heating the slurry from 60 C. to 90 C., adjustof zinc oxide, the amount of zinc oxide coating being from ing the pH of the slurry to at least 10 by adding an alkali to h penfentages on the Wclght of metal compound, adding a soluble zinc salt to said aquesald titanium d1ox1de, said composltlon prepared by forlnous Slurry over a period of at least 5 minutes While i ing an aqueous slurry of euhedral Ti0 particles, heating taining the pH to at least 10, adding a h l l d lt the ShllTY from t0 -r adjusting the P of the to said aqueous slurry over a period of at least 15 minutes slurry to at least 10 y adding an alkali metal compound, while maintaining the pH to at least 10, the amount of adding a soluble Zhlh Salt t0 Said aqueous y Over a said zinc salt being from 1.0% to 4.0% calculated as Period of at least 15 minutes While maintaining the P zinc oxide and based on the weight of said titanium dioxto at least 10, holding the mixture for at least 30 minutes ide, and the amount of said lead salt being from 0.0% at a pH of at least 10, then lowering the pH at a rate of to 1.0% calculated as lead oxide and based on the weight no more than 0.5 pH unit per 10 minute intervals by of said titanium dioxide, holding the mixture for at least 30 minutes at a pH of at least 10, then lowering the pH at a rate of no more than 0.5 pH unit per 10 minute intervals by adding an acid until the pH is lowered to at least 7.5, then filtering, washing and drying said coated product.

References Cited UNITED STATES PATENTS 3,561,968 2/1971 Dantro 9688 8 FOREIGN PATENTS 567,934 3/1945 Great Britain.

US. Cl. X.R. 

